Stents have become extremely important devices in the treatment of cardiovascular disease. A stent is a small mesh “scaffold” that can be positioned in an artery to hold it open, thereby maintaining adequate blood flow. Typically a stent is introduced into the patient's system through the brachial or femoral arteries and moved into position using a catheter and guide wire. This minimally invasive procedure replaces surgery and is now used widely because of the significant advantages it offers for patient care and cost.
In order to deploy a stent, it must be collapsed to a fraction of its normal diameter so that it can be manipulated into the desired location. Therefore, many stents and guide wires are made of an alloy of nickel and titanium, known as nitinol, which has the unusual properties of superelasticity and shape memory. Both of these properties result from the fact that nitinol exists in a martensitic phase below a first transition temperature, known as Mf, and an austenitic phase above a second transition temperature, known as Af. Both Mf and Af can be manipulated through the ratio of nickel to titanium in the alloy as well as thermal processing of the material. In the martensitic phase nitinol is very ductile and easily deformed, while in the austenitic phase it has a high elastic modulus. Applied stresses produce some martensitic material at temperatures above Af and when the stresses are removed the material returns to its original shape. This results in a very springy behavior for nitinol, referred to as superelasticity or pseudoelasticity. Furthermore, if the temperature is lowered below Mf and the nitinol is deformed, when the temperature is raised above Af it will recover its original shape. This is described as shape memory.
Stents having superelasticity and shape memory can be compressed to small diameters, moved into position, and deployed so that they recover their full size. By choosing an alloy composition having an Af below normal body temperature, the stent will remain expanded with significant force once in place. Remarkably, during this procedure the nitinol must typically withstand strain deformations of as much as 8%.
Stents and similar intraluminal devices can also be made of materials like stainless steel and other metal alloys. Although they do not exhibit shape memory or superelasticity, stents made from these materials also must undergo significant strain deformations in use.
FIG. 1 illustrates one of many stent designs that are used to facilitate this compression and expansion. This design uses ring shaped “struts” 12, each one having corrugations that allow it to be collapsed to a small diameter. Bridges 14, a.k.a. nodes, that also must flex in use connect the struts. Many other types of expandable geometries, such as helical spirals, braided and woven designs and coils, are known in the field and are used for various purposes.
One disadvantage of stents made from nitinol and many other alloys is that the metals used often have low atomic numbers and are, therefore, relatively poor X-ray absorbers. Consequently, stents of typical dimensions are difficult or impossible to see with X-rays when they are being manipulated or are in place. Such devices are called radio transparent. There are many advantages that would result from being able to see a stent in an X-ray. For example, radiopacity, as it is called, would result in the ability to precisely position the stent initially and in being able to identify changes in shape once it is in place that may reflect important medical conditions.
Many methods are described in the prior art for rendering stents or portions of stents radiopaque. These include filling cavities on the stent with radiopaque material (U.S. Pat. Nos. 6,635,082; 6,641,607), radiopaque markers attached to the stent (U.S. Pat. Nos. 6,293,966; 6,312,456; 6,334,871; 6,361,557; 6,402,777; 6,497,671; 6,503,271; 6,554,854), stents comprised of multiple layers of materials with different radiopacities (U.S. Pat. Nos. 6,638,301; 6,620,192), stents that incorporate radiopaque structural elements (U.S. Pat. Nos. 6,464,723; 6,471,721; 6,540,774; 6,585,757; 6,652,579), coatings of radiopaque particles in binders (U.S. Pat. No. 6,355,058), and methods for spray coating radiopaque material on stents (U.S. Pat. No. 6,616,765).
All of the prior art methods for imparting radiopacity to stents significantly increase the manufacturing cost and complexity and/or render only a small part of the stents radiopaque. The most efficient method would be to simply apply a conformal coating of a fully dense radiopaque material to all surfaces of the stent. The coating would have to be thick enough to provide good X-ray contrast, biomedically compatible and corrosion resistant. More challenging, however, it would have to be able to withstand the extreme strains in use without cracking or flaking and would have to be ductile enough that the important thermomechanical properties of the stent are preserved. In addition, the coatings must withstand the constant flexing of the stent that takes place because of the expansion and contraction of blood vessels as the heart pumps.
Physical vapor deposition techniques, such as sputtering, thermal evaporation and cathodic arc deposition, can produce dense and conformal coatings of radiopaque materials like gold, platinum, tantalum, tungsten and others. Physical vapor deposition is widely used and reliable. However, coatings produced by these methods do not typically adhere well to substrates that undergo strains of up to 8% as required in this application. This problem is recognized in U.S. Pat. No. 6,174,329, which describes the need for protective coatings over radiopaque coatings to prevent the radiopaque coatings from flaking off when the stent is being used.
Another important limitation of radiopaque coatings deposited by physical vapor deposition is the temperature sensitivity of nitinol and other stent materials. As mentioned, shape memory biomedical devices are made with values of Af close to but somewhat below normal body temperature. If nitinol is raised to too high a temperature for too long its Af value will rise and sustained temperatures above 300-400 C will adversely affect typical Af values used in stents. Likewise, if stainless steel is raised to too high a temperature, it can lose its temper. Other stent materials would also be adversely affected. Therefore, the time-temperature history of a stent during the coating operation is critical. In the prior art it is customary to directly control the temperature of a substrate in such a situation, particularly one with a very low thermal mass such as a stent. This is usually accomplished by placing the substrate in thermal contact with a large mass, or heat sink, whose temperature is controlled. This process is known as controlling the temperature directly or direct control. Because of its shape and structure, controlling the temperature of a stent directly during coating would be a challenging task. Moreover, the portion of the stent in contact with the heat sink would receive no coating and the resulting radiographic image could be difficult to interpret.
Accordingly, there is a need in the art for biomedical devices having radiopaque coatings thick enough to provide good x-ray contrast, biomedical compatibility and corrosion resistance. Further, the coating needs to withstand the extreme strains in use without cracking or flaking and be sufficiently ductile so that the thermo-mechanical properties of the device are preserved.